1. Field of the Invention
The present invention relates to a photo-detector, and more particularly to a field effect transistor (FET) type photo-detector which is suitable for use in a light integrated circuit.
2. Related Background Art
As high speed photo-detectors, a pin photo-diode (pin-PD) and an avalanche photo-diode (APD) have been known. However, the pin-PD has no amplification function. The APD, though it has the amplification function, need a high biasing voltage in operation and hence it is not suitable in an integrated circuit.
High speed FET type photo-detectors which solve the above problems have been proposed by Umeda et al. in Papers of The Institute of Electronics and Communication Engineers of Japan, vol. J68-C, p. 263 (1985) and C. Y. Chen et al. in Appl. Phys. Letter. 42, p. 1040 (1983).
FIG. 1 shows a cross-sectional view of a GaAs-MES-FET type photo-detector. In this photo-detector, a non-doped GaAs buffer layer 35 is laminated on a GaAs semi-insulative substrate 36, and an n type GaAs activation layer 34 having a thickness of 0.2 to 0.5 .mu.m is laminated on the buffer layer 35. A source electrode 31, a gate electrode 32 and a drain electrode 33 for inputting and outputting currents are formed on the activation layer 34. When the photo-detector is biased through the electrodes 31, 32 and 33 and an incident light 38 is applied to the surface or cross-sectional plane, a photo-current flows across the drain electrode 33 and the source electrode 31 so that the incident light 38 can be detected.
Such an FET type photo-detector has advantages of relatively low biasing voltage, a built-in amplification function and applicability to a driver or an amplifier of an integrated semiconductor laser (LD).
However, in the photo-detector of the structure shown in FIG. 1, both the activation layer and the buffer layer are made of GaAs and have the same band gap. Accordingly, when a light having a larger beam diameter than the thickness of the activation layer is irradiated to the end plane of the photo-detector, the light is irradiated to the activation layer as well as the buffer layer so that it is absorbed by the buffer layer, too. In the buffer layer, electron-hole pairs are generated due to the light absorption and the carriers are diffused and reach a depletion layer 37 in the activation layer or are recombined before they reach the depletion layer. As a result, a time response characteristic is deteriorated. Further, the confinement of the carriers in the activation layer which functions as a channel is not sufficient but the carriers leak to the buffer layer and the substrate. As a result, a pinch-off characteristic is deteriorated and an S/N ratio is lowered and a sensitivity is lowered.